The principal research objective
of this project is to elucidate the role of the high-surface area oxide catalyst
support in modulating the selectivity and activity rhodium (Rh) catalysts in
syngas (CO + H2) conversion to ethanol. This is a variant of the conventional
Fischer-Tropsch process, generating long-chain liquid
hydrocarbon oxygenates from gasified biomass / coal. Since thermodynamics
favors the formation of methane and water, generation of alternative (more
useful) products (like ethanol) must necessarily exploit subtle modifications
of reaction kinetics enabled by catalyst modification. Experimentally, it has
been found that changes in the oxide support from silica (SiO2) to titania (TiO2) yields significant increases in the production of C2+
oxygenates. Thus in this reaction, as in many others, the support modulates
activity either directly (by catalyzing reactions on the oxide or at the nanoparticle/oxide
interface) or indirectly (by modifying the electronic structure of the metal
nanoparticle itself, or nanoparticle geometry.) The ultimate goal of our
research is to understand the detailed role of the oxide support in this specific reaction, and also develop a
more comprehensive understanding on support effects in general.

We are using state-of-the-art
computational chemistry methods to explore the role of the oxide support in
various elementary steps involved in syngas conversion. In particular, based on
experimental STM and EXAFS data, we modeled the catalyst as a hemispherical Rh37
cluster, either: in the gas phase (as a model of an unsupported nanoparticle);
or supported on an explicit SiO2 surface; or on TiO2. We
also compare with a Rh(111) single crystal surface.
The direct comparison of these four related systems allows us to separate geometric effects (changes in
nanoparticle geometry induced by the support), from indirect effects (modification of the nanoparticle electronic
structure), from direct effects (for example,
reaction at interfacial sites at the nanoparticle/support interface). A picture
of the model system with an adsorbed CO is shown below. In
all cases, we evaluate our computational model using plane wave density functional
theory (PWDFT) calculations using the VASP software package.

Nearly all previous computational
heterogeneous catalysis calculations neglect the support, treating the reaction
as if it took place on a simple single-crystal metal surface. As such, these
calculations are intrinsically unable to address support effects. The present
calculations are, to the best of our knowledge, the first to attempt to address
the role of the support in Rh-catalyzed syngas conversion. Nonetheless, the
calculations are extremely computationally expensive. As such, we previously
obtained a preliminary 50,000 CPU-hour allocation on the NSF Teragrid (a collection of supercomputers). Just last week,
our request for an additional 860,000 CPU-hour allocation was approved, which
should dramatically enhance our ability to efficiently conduct these
calculations.

We have divided our study into
two phases. In phase one (nearly complete), we examined
the role of the support in modifying the thermodynamics
of the syngas conversion process; in phase two (partially underway), we will
complete our study by examining the modifications to reaction kinetics. In our study of syngas
conversion thermodynamics, we first developed optimized models for the various supported
and unsupported nanoparticles, as described above. We subsequently identified
all kinetically relevant reactants, products and intermediates leading from
CO/H2 to alkane/oxygenate products, including their most stable
binding sites and binding energies on the both the supported and unsupported
nanoparticles. Here, our preliminary data shows that binding of nearly all
species is very similar between the unsupported Rh37 nanoparticle and
the single-crystal Rh(111) surface. As such, geometric changes in the nanoparticle are
unlikely to contribute to changes in the reaction thermodynamics. Nonetheless,
we see significant binding energy differences between adsorbates
on the unsupported vs. supported nanoparticles, and also significant
differences between silica and titania
supports. These changes persist even when the adsorbate is bound to the "top"
of the nanoparticle, rather far from the nanoparticle/support interface. As such, it appears that the support is
modulating the thermodynamics indirectly by modifying the electronic structure
of the nanoparticle itself. We are currently using a variety of analysis
approaches, including projected densities of states, Bader analysis, and
Natural Bond Order analysis to understand the mechanism through which these
support effects are mediated. Corresponding studies of kinetics are also in
preliminary stages. We have identified several transition states for kinetically-relevant
reaction steps, and we will contrast the activation energies for the various
supported and unsupported nanoparticles.

The bulk of this research has
been conducted by Glen Jenness, a postdoctoral
associate in my group who is funded via this grant. Glen came with extensive
(but rather narrow) experience in computational chemistry but no specific
experience in PWDFT or materials science / catalysis experience. The complementary
skill set acquired during his time in my group will be a big asset for Glen and
will contribute to his marketability when he applies for positions at a
national lab (his ultimate career goal). I have also worked extensively with
Glen to help him continue to grow a scientist. In particular, I have spent a great
deal of time help Glen to think deeply in terms of our computational analysis
and allowing him to play an integral role in guiding the evolution of our work.
Both of these are essential skills for an independent scientist.

Although not directly funded by
this grant, Glen works in close coordinate with 3rd year graduate student
Benjamin Dunnington. Benjamin is working to develop
new PWDFT analysis approaches that allow us to probe the bonding and reactivity
of solid-state catalytic systems in novel manners. Benjamin has developed an
extension of the Natural Bond Orbital (NBO) analysis technique to PWDFT
calculations on catalytically-relevant materials. NBO allows us to interpret bonding in complex
bulk or interfacial catalyst systems in simple general chemistry "Lewis-like"
terms. We recently applied these ideas to several simple catalyst model
systems; Benjamin and Glen are coordinating to apply these ideas to obtain a
more "chemical" understanding of the catalysis that is taking place over our
supported Rh nanoparticles.